Low flux density ferromagnetic material



3,055,182 LGW FLUX DENSEITY FERROMAGNETIC MATEREAL This invention relates to magnetic ferrite materials having substantially square hysteresis characteristics and generally referred to as ferrospinels, and relates particularly to an improved composition of such materials exhibiting low flux density.

Ferrospinel bodies are employed as magnetic memory elements and as pulse transfer elements in computers and other data processing apparatus. As is well known in the art, these materials are desirably possessed of a substantially square hysteresis loop characteristic and of a high degree of magnetic retentivity. The patent to Greenhalgh, No. 2,872,666, discloses apparatus employing cores of this type. In this type of apparatus, the magnetic cores are driven to alternate magnetic states at high rates of operation and overheating of the cores is, in some instances, a limiting factor in the speed of operation of the apparatus.

it is the principal object of this invention to provide a material having a suitably square hysteresis characteristic which will have high magnetic retentivity and low flux density, and, accordingly, will provide low heating and thus permit higher rates of core operation.

A further benefit of low flux density materials is that their low demagnetization factor makes possible their employment in improved core configurations other than the conventional toroidal form.

The well known square loop core compositions employing magnesium-manganese-ferrite systems have flux densities in the vicinity of 2000 gausses. When these compositions have their constituents modified, or their process treatments modified, the flux density decreases gradually until minimum values, substantially above 1000 gausses, are obtained, and thereafter with loss of squareness the flux densities drop olf rapidly to a non-magnetic state. A

Efforts have been made to reduce cores to minimum size in order to reduce drive currents, heating and back voltages, however, in single element cores, there is a physical limit to the core size reduction determined by the diameters of conductors which must thread the cores and by various obvious handling problems. Accordingly, the only practical approach to reduction of core flux densities is to reduce the flux density characteristic of the material from which the core is formed.

It is the principal object of this invention to provide magnetic ferrite materials having suitably square hysteresis characteristics which will have a low flux density and thus give rise to low heating and accordingly permit higher rates of core operation.

The flux density of a magnetic material effects, upon the switching thereof, the amplitude of the output voltage pulse resulting from the switching and the amplitude of a back voltage pulse appearing in the drive lines. The sensing and amplifying of an output pulse in most core arrays presents no particular problem, however, the reduction of the back voltage is particularly desirable for the reason that lower drive currents can be employed when the back voltage is reduced.

An additional advantage obtained in cores having low flux density and thus producing less heating than cores of higher flux density is the greater voltage output stability over ranges of rates of core operation.

ited States ate ice It is therefore a further object of this invention to produce a magnetic ferrite material having temperature stability over a wide range of temperatures and affording relatively low back voltages in the drive lines.

It is a further object of the invention to provide a low flux density material having a substantially square hysteresis characteristic and having a high magnetic retentivity.

It is a further object of the invention to provide a low flux density material having a substantially square hysteresis characteristic and having a relatively high Curie temperature, thus being capable of operation at relatively high temperatures without loss of magnetic retentivity.

The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention.

This invention relates to a lithium-titanium-manganeseferrite material prepared from inorganic salt mixtures having the constituents in proportions as will be hereinafter described in connection with Table I set forth below. In the preparation of cores from these materials, powders of the material are prepared in the manner well known in the core art and finely mixed powders of desired proportions are calcined at temperatures ranging from 650 C. to 1000 C. Thereafter, cores are pressed from these powders to densities of approximately 2.5 to 3.5 grams per cc. and sintered at temperatures ranging from 1100 C. to 1400 C. for time intervals ranging from 30 minutes to several hours. These processes are all well known in the art and need not be described in detail herein. it is sufficient to note that the test data set forth in the Table I represents core compositions which were calcined at approximately 800 C. and which were sintered at approximately 1200" C. for approximately 30 minutes. After sintering, the cores were furnace cooled to a temperature of approximately 900 C. and thereafter metal plate quenched to room temperature.

TABLE I Titanium-Lithium-Manganese-Ferrite in M ol Percent F0103 M1100 LigCOa TiOz H Br 13,, Br/Bg 39 21 20 20 3. 2 700 1, 100 0. 635 39 16 20 25 2. 25 655 1, 000 O. 655 39 13 20 1. 500 550 0. 91 39 11 20 30 1. 8 485 500 O. 97 39 9 20 32 1. 425 482 0.86 39 6 20 35 1.90 225 275 0 83 39 O 20 41 2. 35 180 240 O. 75

Each of the composition examples listed in Table I is defined in terms of the mol percent of the powdered materials which were mixed together to produce th resultant sintered composition. It will be noted that several of the examples in the table appear in several groups. This is done in order to facilitate interpretation of the data.

The coercivity values, H listed in the table, are derived by conventional methods well known in the art. The

B values listed in the table represent the flux density at saturation. The E values represent the retained flux density after driving forces have been relieved. The B/B values represent the ratio of retained flux density to saturation fiux density and thus indicate the degree of retentivity of the materials. Suitable retentivity is exhibited by materials having a B /B ratio of .70 or above.

Among the examples listed, it Will be noted that the optimum B /B ratio with low B value is provided by composition 14 having 20 mol percent Li CO 30 mol percent TiO 11 mol percent MnCO and 39 mol percent Fe O Composition 14 also gave hysteresis squareness suitable fo coincident current operation. The compositions of 15, 16, and 17 while having good retentivity and low flux density had insufficient hysteresis squareness for coincident current operation, thus, these compositions are suitable only for bias current type of operation.

It will be evident that the percentage constituents in the optimum composition can be varied and still provide square hysteresis compositions having low flux density and good retentivity. The intermediate range of composition percentage of Fe O is from 35 mol percent to 43 mol percent as is indicated by tests 18 and 19.

The minimum percentage of Fe O is approximately 30 mol percent. Below this value, the composition loses its magnetic properties. The maximum Fe O content is approximately 45 percent. Above this value of iron, the retentivity falls off and the flux density becomes too high. These limitations of Fe O content are known in the art.

The intermediate range of manganese oxide is approximately 7 to 41 mol percent. These values are indicated in tests 19 and 9 respectively.

The lower limit for MnCO is approximately '0 mol percent as shown by composition 17. The upper limit for MnCO is approximately 46 mol percent as shown by composition 8. Compositions having over 46 mol percent exhibit excessive flux density.

The exact eltects, ratio and ranges of Li CO and TiO are dilficult to ascertain without performance of a vast number of composition tests, however, it will be evident from the test data of Table I that the maximum Li CO range for compositions having acceptable flux density and retentivity is approximately 5 to 30 mol percent and the intermediate range for good flux density and retentivity is approximately 10 to 25 mol percent.

Similarly, from the test data, it will be evident that the maximum TiO range is from approximately 10 to 35 mol percent and the intermediate range is approximately 15 to 32 mol percent. It will be obvious to those skilled in the art that the titanium constituent is replaceable by other tetravalent elements from the group consisting of zirconium, tin, and silicon.

The upper limits of titanium and lithium are established by the fact that these ions go to octahedral sites in the cubic lattice and therefore decrease the magnetic moment 4% of the material, decrease its flux density, and ultimately decrease its B /B ratio.

From compositions 1 to 4 employing percent TiO and from composition employing 0 percent Li CO it Will be evident that one of these materials alone will not produce the low flux density and high retentivity produced when both the materials are employed.

Referring again to Table 1, compositions 18, 14 and 19 all display a Curie temperature of approximately 260 C. This Curie temperature is sufficiently high to permit operation of the cores in the vicinity of 100 C. Thus, a core array constructed from the cores of this composition may be operated at reasonably high temperatures without any danger of loss of data therein.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A rectangular hysteresis loop ferrospinel system formed by heating a mixture consisting essentially of:

7 to 46 mol percent MnCO 5 to 30 mol percent Li CO to 35 mol percent TiO and, 30 to 45 mol percent Fe O 2. A rectangular hysteresis loop ferrospinel system formed by heating a mixture consisting essentially of:

7 to 41 mol percent MnCO 10 to 25 mol percent Li CO to 32 mol percent TiO and, 35 to 43 mol percent Fe O 3. A rectangular hysteresis loop ferrospinel system formed by heating a mixture consisting essentially of:

11 mol percent MnCO mol percent Li CO mol percent TiO and, 39 mol percent Fe O References Cited in the file of this patent UNITED STATES PATENTS 2,736,708 Crowley et a1 Feb. 28, 1956 2,847,101 Bergmann Aug. 12, 1958 2,851,419 Gorter et al. Sept. 9, 1958 FOREIGN PATENTS 287,007 Switzerland Mar. 2, 1953 OTHER REFERENCES Kordes et al.: Chemical Abstracts, vol. 46, col. 4411,

: May 25, 1952, 

1. A RECTANGULAR HYSTERSIS LOOP FERROSPINEL SYSTEM FORMED BY HEATING A MIXTURE CONSISTING ESSENTIALLY OF: 